Cask means container. The Histories of Herodotus, written in 424 BC, refers to "casks of palm-wood filled with wine" being moved by boat to Babylon, though clay vessels would also have been used. Stout wooden barrels held together with an iron hoop were developed by the north European Celts during the Iron Age for storing goods. Over the centuries other methods have been developed for preserving and storing beer but this method is still used, particularly in Britain.

Bottled beers were commonplace by the 17th century for the well off who wished to drink outside of public inns, or who wanted to take a beer with them when fishing. Such as the famous story of Alexander Nowell, the Dean of St Paul's, who, in 1568, left his bottled beer by the river bank, and upon returning a few days later discovered the bottle opened with a bang and that the contents were very tasty. But while the middle and upper classes could indulge themselves with such expensive luxuries, the ordinary folk continued to drink their beer served direct from the cask. India Pale Ale (IPA), the famous ale that was shipped to India was delivered in casks, and only transferred to the bottle for the civilian middle classes; the troops drank their beer the same way they drank it back home, from flagons filled direct from the cask. But as beer developed and became paler and lower in alcohol, so it became more difficult to keep it fresh tasting in the cask, especially in countries with warmer climates. By the late 19th century commercial refrigeration and Louis Pasteur's flash heating method of sterilisation prolonged the life of beer. In Britain's cooler climate these methods did not catch on at this time.

Not all beer in mainland Europe is pasteurised; there are plenty of examples of unfiltered, unpasteurised beers, but these will commonly be served from a chilled container under pressure: a keg.

Traditionally draught beer came from wooden barrels, also called casks. In the 1950s these began to be replaced by metal casks of stainless steel or aluminium, mainly for quality reasons as they could be sterilised and the beer was therefore less likely to spoil, but also for economic reasons. An additional benefit of the switch to metal casks was that staling from oxygen in the air could be reduced. Subsequently, in the early 1960s a form of metal cask, known as a keg, was introduced which allowed for more efficient cleaning and filling in the brewery.

The essential differences between a traditional cask and a keg are that the latter has a centrally located downtube and a valve that allows beer in and gas out when filling and vice versa when beer is dispensed. Also kegs have a simple concave bottom whilst the barrel or cask design allowed sediment to be retained in the cask. This aspect of keg design meant that all the beer in the keg was dispensed which therefore required that the beer be processed by filtration, fining or centrifuging, or some combination of these, to prevent sediment formation. Lastly, kegs have straight sides unlike the traditional barrel or cask shape. In order to get the beer out of a keg and into a customer's glass, it can be forced out with gas pressure, although if air or gas at low pressure is admitted to the top of the keg it can also be dispensed using a traditional hand pump at the bar.

By the early 1970s most beer in Britain was keg beer, filtered, pasteurised and artificially carbonated, and most British brewers used carbon dioxide for dispensing keg beers. This led to beers containing more dissolved gas in the glass than the traditional ale and to a consumer demand for a return to these ales. By contrast, in Ireland, where stout was dominant, the use of a mixture of carbon dioxide and nitrogen for dispensing prevented the beer from becoming over-carbonated. Rare examples of natural beers could still be found in the farmhouse beers of Northern Europe and the maize beers of South America for example, in essence the last great stronghold of natural beer was about to be wiped out. In 1971 the Campaign for Real Ale (CAMRA) was founded in Britain to save what they came to term "real ale".

Real ale is the name coined by the Campaign for Real Ale (CAMRA) in 1973 for a type of beer defined as "beer brewed from traditional ingredients, matured by secondary fermentation in the container from which it is dispensed, and served without the use of extraneous carbon dioxide".[2][3] The heart of the definition is the maturation requirements. If the beer is unfiltered, unpasteurised and still active on the yeast, it is a real ale; it is irrelevant whether the container is a cask or a bottle.

This is beer which is commonly regarded as Real Ale, but contains much less yeast and has a very short shelf life - two or three days for a typical cask. It is obtained by decanting, or re-racking, Real Ale from one cask (or other container at the brewery) to another, thus leaving behind the majority of the (already residual) yeast. The ale in the first cask/container will (usually) have already been fined, taking the majority of the yeast to the bottom of the cask. Ale should only be re-racked immediately before delivery to the point of service.

Cask ale usually has finings added which drag the yeast to the bottom; when the finings have cleared the beer it is said to have "dropped bright" and the beer will look clear rather than cloudy. However, if a beer has been filtered, or has been cleared of yeast by using finings, and then "racked"—transferred to another container—this is "bright" or re-racked beer. Bright beer is essentially unpasteurised beer that has been cleared of yeast and placed in a different container. It no longer sits on the yeast. As such, strictly speaking, it is not real ale because it cannot continue to ferment in the container in which it now finds itself.

There is a significant difference between Real Ale that has "dropped bright" and "bright beer".

The expression "bright beer" is commonly used, particularly by older established breweries, for any filtered and pasteurised beer. However, the expression "re-racked beer" should be reserved for beer which has been racked off (decanted) from a cask of cask-conditioned beer immediately before delivery to the venue where it will be served. It is often regarded as "real ale" because it does still contain some residual yeast, albeit a very small amount, and is otherwise handled exactly as is real ale. Because there is only a very small amount of yeast, there is very little secondary fermentation, and re-racked beer has a very short shelf life of two or three days.

The fundamental distinction between real and other ales is that the yeast is still present and living in the container from which the real ale is served, although it will have settled to the bottom and is usually not poured into the glass. Because the yeast is still alive, a slow process of fermentation continues in the cask or bottle on the way to the consumer, allowing the beer to retain its freshness. Another distinction is that real ale should be served without the aid of added carbon dioxide, or "top pressure" as it is commonly known. Common dispensing methods are the handpump, or "by gravity" direct from the cask. Electric pumps are occasionally seen, especially in the Midlands and Scotland.

The cask breather is an alternative to a hard spile and proper cellar management, and as such is not recognised by CAMRA.

When a cask has been tapped, the beer starts to come into contact with oxygen—and a beer in contact with oxygen has a limited life. Stronger beers last longer, but for most ales with an ABV in the low 4% region, three days is typical.[citation needed] However, if proper cellar management is practised, including "hard spiling" beer between sessions, almost any genuine real ale should last around a week. If the pub doesn't have a high turnover, or if a beer is not popular, such as dark milds, three days will not be enough to sell all the beer in the cask. A cask breather allows a small amount of CO2 to replace the oxygen in the cask. Not enough CO2 to enter the beer[citation needed] or push it up to the bar—that's "top pressure"—but enough "blanket pressure" to keep the beer fresh tasting for longer by replacing some or most of the oxygen that has made it into the cask with CO2, an inert gas.

The use of cask breathers is considered "extraneous carbon dioxide", so CAMRA does not endorse this method.[4]

Broadly speaking, cask ale brewing starts the same as that of keg beer. The same brew run could be used to make cask, keg, and bottled beer. The difference is what happens after the primary fermentation is finished and the beer has been left to condition. Typically keg and bottled beers are either sterile-filtered or pasteurised or both, but beer destined for cask is simply 'racked' (poured) into the cask in its natural state. Finings are usually placed in the cask to assist 'dropping' the yeast giving a clear beer. Extra hops and priming sugar may also be added. The cask is sealed and sent off to the pub. In this state it is like a bottle-conditioned beer and, like bottle-conditioned beers, the beer will continue to develop for a certain period of time. Also like bottle-conditioned beers, the length of time the beer can last in the cask will depend on the nature of the beer itself: unopened, stronger beers can last for months; light, delicate beers need to be tapped and sold quickly. Stronger beers may also need longer to settle and mature. Some pubs have been known to keep very strong beers in a sealed cask for a year or more to allow them to fully develop.

When the landlord feels the beer has settled, and they are ready to serve it, they will knock a soft spile into the shive on the side of the cask. The major difference in appearance between a keg and a cask is the shive. A keg does not have a shive on the side. The majority of casks these days are metal, and look similar to a keg, but with the rounded traditional barrel shape (kegs are often straight-sided). Even though there are still some wooden casks around, these are rare; in fact there are more plastic casks around than wooden ones. Plastic casks are increasing in popularity because they are cheaper to buy and lighter to carry. Though they don't last as long, they are also less likely to be stolen as they have no melt-down value. Beer casks come in a number of sizes, but by far the most common in the pub trade are those of 9 gallons (72 pints or roughly 41 litres) which is known as a Firkin and 18 gallons (144 pints or roughly 83 litres) known as a Kilderkin. (N.B. These are imperial gallons, equal to 1.201 US gallons each.)

The soft spile in the shive allows gas to vent off. This can be seen by the bubbles foaming around the spile. The landlord will periodically check the bubbles by wiping the spile clean and then watching to see how fast the bubbles reform. There still has to be some life in the beer otherwise it will taste flat. When the beer is judged to be ready, the landlord will replace the soft spile with a hard one (which doesn't allow air in or gas out) and let the beer settle for 24 hours. They will also knock a tap into the end of the cask. This might simply be a tap if the cask is stored behind the bar. The beer will then be served simply under gravity pressure: turn on the tap, and the beer comes out. But if the cask is in the cellar, the beer needs to travel via tubes, or beer lines, and be pumped up to the bar area, normally using a handpump also known as a 'beer engine'.

Cask ale in pubs is usually served with a beer engine or handpump, which is used to siphon the beer from the cellar. The beer engine is a 0.5 imp pt (0.28 l), sometimes 0.25 imp pt (0.14 l), airtight piston chamber; pulling down on the handle raises the piston which drags up a half pint of beer. When a cask is first tapped into the beer engine, or after the lines have been washed through, the pump needs to be pulled several times to clear the lines of air or water. The line will continue to hold beer, which will tend to go stale overnight, so the first beer of the day pulled through will be thrown away. Most pubs will pull through at least a pint of beer on each beer engine before they open, while others will wait for the first order of beer on that pump. Experienced bar staff will serve a pint with long, smooth, slow pulls of the pump handle, plus a short final pull to make sure the glass is full.

A small flip tap and a short spout is the standard neck for dispensing cask ale. An alternative is a long spout with a tight 180° turn, called a swan-neck, which is designed to force the beer into the glass, agitating it so that a fuller head is created. Some drinkers disapprove of swan-necks, believing that flavour is reduced.[5] In some pubs a small device or cap, known as a sparkler, is fitted to the end of the spout and acts like a sprinkler at the end of a hose pipe.[6] This can be twisted to regulate the flow of the beer. When the sparkler is tight, the beer is severely agitated resulting in a creamy head; it is softer and creamier with less bitterness.[citation needed]

It is also considered proper to dispense beer directly from the cask, as in pubs which have a tap room rather than a cellar. Gravity dispense is often used in beer festivals as well.

Some pubs disguise keg beer by having an imitation pump handle on the bar. If the bar staff have merely turned on a tap, or are just resting their hand on a very small handle with no pump action, then this is a keg beer. Exceptions are some pubs (in the north and occasionally elsewhere) which use electric pumps or the pubs in Scotland that use traditional air-pressure founts[7] on cask ale.